1. Field of the Invention
The present invention relates to the structure of a multiple quantum well (referred to as MQW hereinafter) semiconductor laser, and more particularly to the structure of a long wavelength MQW semiconductor laser having InP as the substrate, and used as a light source for optical communication or the like.
2. Description of the Prior Art
Research and development on the semiconductor lasers employing MQW in the active layer are being pursued vigorously since they can bring about marked improvement of the laser characteristics, and numerous papers have been published in recent years.
FIG. 6(a) is a schematic perspective view of a conventional 1.48 .mu.m band MQW semiconductor laser chip disclosed in IEEE Photonics Technology Letters, Vol. 3, pp. 415-417, 1991, and FIG. 6(b) is a band diagram of its active layer part.
In the figures, 201 is an n-InP substrate, 202 is an MQW active layer, 203 is a p-InP clad layer, 204 is a p-InP block layer, 205 is an n-InP block layer, 206 is a p-InP burying layer, 207 is a p.sup.+ -InGaAsP cap layer, and 208 are electrodes. This semiconductor laser has a buried structure called double-channel planar buried heterostructure (DC-PBH), and its active layer has a width of 1.6 .mu.m. In addition, 211 is a 1.15 .mu.m composition InGaAsP optical waveguide layer, 212 is a 1.15 .mu.m composition InGaAsP barrier layer, 213 is an InGaAs quantum well layer, 214 is a 1.15 .mu.m composition InGaAsP optical waveguide layer on the p side.
Here, the number of quantum well layers is set to be two to seven. FIG. 6(b) shows the 5 layer case. The thickness of the quantum well layer is 4 nm, the thickness of the barrier layer is 13 nm, and the thickness of the waveguide layer combining the quantum well layers, the barrier layers and the optical waveguide layers is 200 nm. With the above constitution, when the number of quantum well layers is changed from two to seven, the internal absorption loss changes from 5 to 10 cm.sup.-1, but the internal differential quantum efficiency remains at a low level of 60 to 70% or so. In this example, the energy difference (referred to as .delta.Ev hereinafter) between the first quantum level of the heavy hole of the quantum well and the top of the valence band of the barrier layer is about 180 meV, and the energy difference (referred to as .delta.Ec hereinafter) between the first quantum level of the electrode of the quantum well and the bottom of the conduction band of the barrier layer is about 80 meV.
In a semiconductor laser having a structure in which carriers are injected into the quantum well layers from the direction traversing the quantum well layers and the barrier layers, it is conjectured that the internal differential quantum efficiency shows a small value because of the occurrence of a nonuniform injection of the holes when .delta.Ev has such a large value as 180 meV. Further, when the number of quantum well layers is such a small value as two, the optical confinement factor is small so that even though the internal loss may be as small as 5 cm-1, the gain also becomes low. This results in an increase in the lasing threshold when one forms a laser with short cavity resonators.
In the design of an MQW semiconductor laser, there are an extremely large number of parameters such as the quantum well composition, the lattice distortion of the quantum well, the thickness of the quantum well layer, the number of the quantum well layers and the barrier layer composition, etc. Accordingly, to optimize the quantum well structure for a certain lasing frequency, it is necessary to repeat experimenting in a trial and error fashion which gives rise to a problem that an enormous amount of labor and cost are required, and that the research and development has to be extended over a long period of time.